The present invention relates to a commutation device for a brushless d-c motor and more particularly to a commutation device which provides regulated current to the stator windings as a function of rotor position by means of a regulator circuit.
Brushless d-c motors may be controlled by a commutation device which includes a transistorized bridge circuit for connecting the d-c voltage source to the stator windings as a function of rotor position. Use of a transistorized bridge circuit allows current to be supplied to the windings in both directions so that operation of the motor in two directions is possible. Brushless d-c motors of this type may advantageously be used as stepping motors, and operating such motors at constant current provides additional advantages.
In commutation devices which include a regulator circuit to provide constant current to the bridge circuit, a control element such as a main control transistor is provided as the output of the regulator circuit and is connected between the voltage source and the bridge circuit. The regulator circuit may include a comparison element such as a level detector to which are supplied the sampled and reference values of current and which controls operation of the main transistor, there being a hysteresis involved with switching the main transistor off and on. Where large currents are supplied to the motor, Darlington-connected transistors are used in the bridge circuit for commutation instead of simple transistors since they provide a large output current. However, the capacitances of Darlington-connected transistors add up and when such transistors are used in the commutation device, a charge reversal in the transistor capacitances occurs each time the main control transistor is driven into conduction. Brief current peaks due to the respective cut-off transistors of the bridge circuit are generated superimposed on the current supplied to the motor. These peaks can impair the function of the level detector of the regulator circuit if the magnitude of the current peaks at the level detector inputs is higher than the level set for the level detector. In such a case, operation of the level detector and regulator circuit are unstable. The current peaks may be suppressed at least partially by smoothing (filtering), particularly if transistors are used. However, the current hysteresis depends on the motor operating voltage and time constant. Therefore, operation of the regulator circuit and level detector is not stable for different motor voltages and motor types and such commutation devices cannot, per se, be used for different motors and voltages.